As an exhaust gas purification system for purifying NOx in exhaust gas of a diesel engine, an SCR (Selective Catalytic Reduction) system using an SCR device has been developed.
The SCR system supplies urea water to an exhaust gas upstream of SCR, generates ammonia by the heat of the exhaust gas, and reduces and purifies NOx on an SCR catalyst by the ammonia (see, for example, Patent Document 1).
However, there is a problem in that urea water cannot be supplied in cold weather because the urea water is frozen at about −10° C. For this reason, the SCR system is mounted with an SCR thawing control system that supplies engine cooling water to a urea water supply system in cold weather and performs a thawing control for thawing frozen urea water.
When any one of a tank temperature inside a urea tank for storing urea water, a temperature inside a urea pipe between the urea tank and a dosing valve for injecting urea water on an upstream side of an SCR device, and an SM (Supply Module) temperature inside a supply module connected to the urea pipe is less than a predetermined threshold value, a conventional SCR thawing control system determines that the urea water has been frozen, and performs a thawing control. Further, in general, because there is no sensor for detecting the temperature inside the urea pipe, an ambient temperature is regarded as the temperature inside the urea pipe.
When the tank temperature or the SM temperature is greater than or equal to the threshold value and a set time according to an ambient temperature has elapsed, the conventional SCR thawing control system determines that thawing has been completed, and operates an SM pump of the supply module to fill urea water into the urea pipe, maintain a pressure thereof at a predetermined value, and inject urea water from the dosing valve (or prepare to be able to inject urea water from the dosing valve).